This invention relates to hollow glazing units comprising vitreous sheets held in spaced facing relationship by marginal connecting means.
Such units have insulating properties. They are extensively used in walls of buildings for thermal insulation purposes. And, depending on their composition, they can be effective for screening off radiation in certain parts of the electromagnetic spectrum.
It is well known that insulating effects of one kind or another can be increased by increasing the number of spaced skins or panes of such a unit. And in the building industry there has for many years been a marked interest in the use of hollow glazing units comprising three or more spaced panes or lights.
In comparison with a double window having two vitreous sheets, a multiple glazing unit comprising three vitreous sheets is thick, and of course it is significantly heavier. The greater weight implies the need for a heavier frame in which to mount the unit. In many circumstances these factors preclude the installation of those more complex units.
Multiple glazing units have been proposed in which two vitreous sheets are glued in spaced relationship to marginal spacing means which also serves as a frame for holding a plastics sheet or foil stretched between those vitreous sheets. By dividing the space between the outer vitreous sheets, the foil reduces convection currents and improves the thermal insulating effect of the unit. Moreover such a foil can usefully modify the radiant energy transmitting characteristics of the unit. For the latter purpose a coloured or tinted foil or a foil having a coating which is coloured or has good heat-reflecting properties can be used.
It is necessary for the foil to retain its truly planar wrinkle-free condition during the useful life of the glazing unit. Otherwise the foil will cause optical distortion. It is therefore important that during use of the glazing unit the support to which the margin of the foil is attached is not itself forced to undergo movements, e.g. as a result of mechanical forces on the unit or thermal gradients, such as to allow sagging or to cause deformation of the foil. On the other hand the marginal coupling of the vitreous sheets must be capable of absorbing such imposed forces without rupture or weakening of the joints between the vitreous sheets and the marginal spacing means.
In the previously proposed constructions of hollow glazing unit incorporating a stretched foil, the vitreous sheets are glued to spacer means which is wide enough to hold an adequate margin of the foil and provides a substantially rigid joint between the vitreous sheets. In general times, the spacers of the said units comprise separate or joined opposed portions forming a kind of jaw in which a margin of the foil is secured. In some units the foil margin is glued into the jaw (cf United Kingdom patent specification GB No. 2 065 756A and European Patent Application No. 0 034 813 Al). In others the spacer portions have mechanical fastening means by which they can be clamped onto the foil margin (cf FIG. 1 of United Kingdom patent specification GB No. 2 011 985 A).